An ace in the hole is an argument, plan or thing kept hidden until needed.

VOA: special.2009.11.22

Their argument: The plan could create chaos on their networks.

FORBES: Spectrum Warriors

Guyana's president, Bharrat Jagdeo, made a different argument as he presented a plan for a low-carbon economy designed to suit the interests of all countries at an early stage of developing their natural resources.

ECONOMIST: Global consensus on climate change is maddeningly elusive

The source of the current argument is the tax-cut plan itself, and the somewhat bizarre way in which it was implemented.

ECONOMIST: Playing politics with tax | The